This invention relates to excitation circuits for Coriolis mass flowmeters which are connected to, are powered exclusively from, and output a measurement signal exclusive via, a two-wire process control loop. In the following, such a mass flowmeter will be referred to as a two-wire mass flowmeter.
Coriolis mass flowmeters were described theoretically long ago and have been known in their currently commercially available form for about 25 years. This form of flowmeter contains a mechanical sensor which comprises at least one straight flow tube or at least one flow tube bent in one plane or three-dimensionally as well as associated electronics. The flow tube is fixed at the inlet and outlet ends. In operation it is excited into vibrations between these fixing points.
Conventional mass flowmeters are power-line-operated devices and must therefore have at least two electric leads, i.e., two wires. The measurement signal representative of, and particularly proportional to, mass flow rate is produced and output according to a standard established for this purpose, such as the 4- to 20-mA current standard, a usual frequency standard, or a digital standard; for this, two further wires are necessary.
In EP-A 10 94 307, the possibility of providing Coriolis mass flowmeters with only two wires, i.e., to design the latter as two-wire mass flowmeters, is mentioned more in passing and only briefly; such two-wire mass flowmeters are not yet on the market.
As can be seen from the aforementioned EP-A, two-wire meters deliver as a measurement signal an output current whose instantaneous values represent a signal provided by a physical-to-electrical transducer as proportionally as possible. The two wires serve both to supply power, for which purpose a DC voltage source must be connected to the two wires from outside, and to transmit the measurement signal.
In the case of two-wire meters according to the above-mentioned 4- to 20-mA current standard, a given current value within this current range corresponds to exactly one measurement-signal value. Only the current range below 4 mA is usable for the supply of power to the electronics of the two-wire meter. As a result, power is available only on the order of 50 mW, which will hereinafter be referred to as xe2x80x9clow powerxe2x80x9d.
In view of these facts, two-wire meters with the aforementioned 4- to 20-mA current standard are particularly suited for use in potentially explosive atmospheres.
Furthermore, two-wire meters are frequently designed to be capable of cooperating with one of the conventional field buses. This can be accomplished by connecting the meter to the field bus directly, e.g., according to the FIELDBUS protocol (FIELDBUS is a registered trademark of FIELDBUS FOUNDATION), or indirectly via a bus coupler, e.g., according to the so-called HART protocol (HART is a registered trademark of the HART User Group).
The aforementioned electronics of Coriolis mass flowmeters comprise an excitation circuit and an evaluation circuit. The excitation circuit serves to vibrate the at least one flow tube through which passes the fluid to be measured.
The vibration occurs at a frequency equal or adjacent to the instantaneous mechanical resonance frequency of the flow tube; one of the parameters determining the resonance frequency is the density of the fluid; the density, in turn, is dependent on the temperature of the fluid. Therefore, conventional mass flowmeters include at least one temperature transducer.
Since the vibrating flow tube serves to measure mass flow ratexe2x80x94this is the fluid mass per unit timexe2x80x94, the evaluation circuit produces a corresponding measurement signal.
The available low power must suffice to supply the excitation and evaluation circuits. An evaluation circuit particularly suited for this purpose is the circuit disclosed in EP-A 10 59 515, corresponding to U.S. patent application Ser. No. 09/579,384, filed May 20, 2000. An excitation circuit requiring so little power has not yet been described in the prior art, however.
It is therefore an object of the invention to provide excitation circuits having a low power consumption and, thus, being suitable for two-wire Coriolis mass flowmeters.
To attain this object the invention provides an excitation circuit for a Coriolis mass flowmeter having at least one vibrating flow tube, an electromechanical excitation assembly for vibrating the at least one flow tube at a frequency equal or adjacent to the instantaneous mechanical resonance frequency of the flow tube; a tranducer assembly for generating a first transducer signal representing inlet-side vibrations of said measuring tube and a second transducer signal representing outlet-side vibrations of said measuring tube: The excitation circuit according to the invention comprises an demodulation stage fed by one of the transducer signals or a sum of both transducer signals, said demodulation stage being operable to generate an output signal representing an oscillation amplitude of said vibrating tube, a comparison stage fed by the output signal of the demodulation stage, said comparison stage being operable to generate an output signal representing a deviation of said oscillation amplitude of the vibrating tube from an predetermined reference oscillation amplitude for said vibrations.
In a first variant of the invention the excitation circuit further comprising an amplitude modulation stage for modulating said signal fed to the demodulation stage with said output signal from the comparision stage, said amplitude stage being operable to generate a drive signal for supplying said excitation assembly.
In a second variant of the invention the excitation circuit further comprising an pulse duration modulation stage for modulating said signal fed to the demodulation stage with said output signal from the comparision stage, said pulse duration stage being operable to generate a drive signal for supplying said excitation assembly.
In a preferred embodiment of the first variant of the invention the demodulation stage comprises a peak detector for detecting the amplitude of said signal fed to the demodulation stage.
In a further preferred embodiment of the invention the demodulation stage comprises a preamplifier for preamplifying said signal fed to the demodulation stage.
In another preferred embodiment of the invention, the comparsion stage comprises an amplifier, an integrating amplifier, or a differentiating and integrating amplifier.
In a preferred embodiment of the first variant of the invention, the amplitude modulation stage comprises a DC/DC converter fed by the output signal of the comparision stage, said converter delivering a DC voltage having an amplitude depending on the output signal from the comparision stage.
In a further preferred embodiment of the first variant of the invention, the output stage comprises a complementary push-pull stage which is supplied with the DC voltage delivered by the DC/DC converter.
In another preferred embodiment of the first variant of the invention, the amplitude modulation stage comprises an output stage with an operational amplifier wired as follows: An inverting input is connected to ground through a first resistor. A noninverting input is connected to the output of the multiplier through a second resistor, which has the same value as the first resistor. An output is connected via a third resistor to a first terminal of a primary winding of a transformer, a second terminal of which primary winding is connected to ground, said transformer having a secondary winding connected to the electromechanical excitation assembly and being a step-up transformer; the inverting input is connected via a fourth resistor to the first terminal of the primary winding; and the noninverting input is connected via a fifth resistor, which has the same value as the fourth resistor, to the output.
In a preferred embodiment of the second variant of the invention, the output stage comprises a complementary push-pull stage which is supplied with a constant positive DC voltage and a constant negative DC voltage.
Furthermore, it is an object of the invention to provide a two-wire Coriolis mass flowmeter which is connected to, is powered exclusively from, and outputs a measurement signal exclusively via, a two-wire process control loop.
To attain this object the invention provides further a Coriolis mass flowmeter comprising an excitation circuit according to the first or the second variant of the invention.
In a preferred embodiment of the Coriolis mass flowmeter according to the invention, the two-wire process control loop carries a DC supply current; in particular, the measurement signal is a direct current, preferably in a range of 4 to 20 mA, or a digital signal, in which case the Coriolis mass flowmeter is connected to a field bus, particularly by means of the two-wire process control loop.